It is often beneficial to coat medical devices so that the surfaces of such devices have desired properties or effects. For example, it is useful to coat medical devices to provide for the localized delivery of therapeutic agents to target locations within the body, such as to treat localized disease (e.g., heart disease) or occluded body lumens. Such localized drug delivery avoids the problems of systemic drug administration, which may be accompanied by unwanted effects on parts of the body which are not to be treated, or because treatment of the afflicted part of the body requires a high concentration of therapeutic agent that may not be achievable by systemic administration. Localized drug delivery is achieved, for example, by coating balloon catheters, stents and the like with a therapeutic agent. The coating on medical devices may provide for controlled release, which includes long-term or sustained release, of a bioactive material.
Aside from facilitating localized drug delivery, medical devices are coated with materials to provide beneficial surface properties. For example, medical devices are often coated with radiopaque materials to allow for fluoroscopic visualization during placement in the body. It is also useful to coat certain devices to achieve enhanced biocompatibility and to improve surface properties such as lubriciousness.
Conventionally, coatings have been applied to medical devices by processes such as dipping, spraying, vapor deposition, plasma polymerization, and electro-deposition. Although these processes have been used to produce satisfactory coatings, there are numerous potential drawbacks associated therewith. For example, it is often difficult to achieve coatings of uniform thickness, both on individual parts and on batches of parts. Also, many of these conventional coating processes require that the coated part be held during coating, resulting in defects such as bare spots where the part was held and thus requiring subsequent coating steps.
There is, therefore, a need for a cost-effective method of coating medical devices that results in uniform, defect-free coatings and uniform drug doses per unit device. The method would allow for a multiple stage coating in order to apply a bioactive material that may be environmentally sensitive, e.g., due to heat and light (including ultra-violet) exposure and due to degradation of the bioactive material due to process-related forces (e.g., shear). The method would, thus, allow for better control of the sensitivity of the bioactive material and reduce any potential degradation due to environmental issues. The method would also reduce variations in the coating properties.